Currently, poles for accumulators are made in lead, generally in standardised forms, at least as regards the contact end. Poles are known that have a tapered contact part and a base part. The latter may be provided with protruding flange, labyrinth, and/or thread-like elements that improve sealing embedding of such base part into the plastic material of the accumulator cover. The two parts are either obtained separately and thereafter joined by soldering or by other suitable methods, or as a one, monolithic element. Eventually, a terminal pole should have a compact structure with no pores that may allow leakage or venting of the accumulator content. Pores also promote early corrosion and oxidation of the pole, limiting thus its conductivity.
Ordinary terminals are made, principally, with the methods known as drop casting, die-casting, roll-forming or pressing.
In the first, drop casting method, the lead is melted and poured into moulds that yield poles of the desired final shape. Proceeding in this way, terminals are obtained however with little resistance to corrosion and prone to causing the seeping of the acid from within the accumulator towards the exterior because of the porosity and the presence of damaging blowholes in the structure of the pole. Thus, efficient seal, between the plastic cover and the pole, is often not obtainable.
In the second method the lead is subjected, in the melted state, to a die-casting process. In this case, the high-pressure die-casting, ensures slight improvements relative to the surface porosity and production velocity. The defects typical of the previous method are nevertheless not completely eliminated.
In the third, roll-forming method shaped, forming rolls are used to obtain terminals with good compaction features. Generally such method and the devices used are employed as a final, additional working operation of the die-casting method. The resulting process therefore is complicated, involves complex additional systems and method steps and has rather low productivity.
In the fourth method the pole is manufactured by pressing, starting from a previously calibrated, generally cylindrical lead element, which therefore always needs preliminary processing and equipment to obtain a diameter and length in required tolerance. This element is placed in a press mould, which gives a semifinished product having a shape similar to the final one, but which has to be finished with further operations in which end parts of the semifinished product are removed. Satisfactory compactness degree, i.e. lack of pores, is achieved but the method ensures rather low productivity and requires very sturdy devices due to the high working stress involved by the deformation process.
Furthermore, different operations are carried out on different machines and the percentage of material that results in the end as manufacturing waste is high, being in the range of 20-25% of the total material employed.
Consequently, the production costs are rather high.